VHDL Made Easy! - Xilinx

44
Carry-Logic
Continued from the previous page
Figure 5
Function Select Flip-Flop
Figures 6(a), left, & 6(b)
Two Peak Detectors
when needed. Thus, it is possible to generate
either A–B or B–A on demand.
Given this capability, the absolute difference
is obtained by simply choosing the
PEAK DETECTOR
In a peak detector, the current peak
value is stored in a register, and is subtracted
from every new input value. If the
difference is positive, the new input is
larger, and it replaces the value in the
register as a new peak. Otherwise, the
register is unchanged.
Only the sign of the difference is of
interest in determining whether the register
is updated or not. The other difference
bits need not be generated, and the corresponding
function generators are free to
be used in controlling the register.
Figure 6(a) shows a typical bit. The
sign of the difference is routed to all bits
to select the value loaded into the register.
This operation includes the sign bit of
the register. Consequently, the subtraction
must be sign-extended by one bit so that
function that yields a positive result. However,
directly feeding back the sign of the
output to select the function will result in
instability, and a flip-flop must be added to
eliminate this possibility (Figure 5).
In the first of two operations on the
same inputs, either subtraction can be
performed. If the first result is negative, the
flip-flop is toggled to select the other function
to achieve a positive result in the second
operation. If the first result is positive,
the flip-flop is not toggled, and the first
operation is repeated. In either case, the
result of the second operation is positive.
the sign of the difference is also available.
The peak detector can be reset by forcing
the sign output to a one. This causes
the current input value to be loaded as a
new peak, regardless of the value of the
previous peak.
The circuit described above is equivalent
to using the sign of the difference to enable
the peak register, and the CLB Enable Clock
pin could be used equally well. However,
the two techniques impose different routing
constraints, and either may be more effective
than the other in different situations.
If enable clock is used, the function
generators can be used for other purposes.
For example, to initialize the peak detector
with a predetermined value that is only
changed by a peak that exceeds it, as in
Figure 6(b). ◆